Question: Complete the recursive formula of the arithmetic sequence $9, -1, -11, -21,...$. $d(1)=$
Solution: The first term is $9$ and the common difference is $-10$. ${-10\,\curvearrowright}$ ${-10\,\curvearrowright}$ ${-10\,\curvearrowright}$ $9,$ $-1,$ $-11,$ $-21,...$ This is the recursive formula of $9, -1, -11, -21,...$ $\begin{cases} d(1)=9 \\\\ d(n)=d(n-1)+(-10) \end{cases}$